1. Field of the Invention
The present invention relates to an information recording and reproducing apparatus, and more particular to a signal processing method, a signal processing circuit and a signal processing apparatus and a signal modulating/demodulating method and a modulating/demodulating apparatus for use in an information recording and reproducing apparatus such as a magnetic disk apparatus.
2. Description of the Prior Art
For signal processing in a magnetic recording and reproducing apparatus such as a magnetic disk apparatus, a partial response maximum likelihood decoding (PRML) system is used. The PRML system is a combination of a partial response PR system and a maximum likelihood (ML) decoding method. The PR system is a technique which allows limitation of the band of codes by positively utilizing interference between adjoining signals. Since the PR system gives rise to correlations between signals, the ML decoding method makes possible decoding on a sequence-by-sequence basis by utilizing the correlations.
FIG. 2 shows the configuration of a magnetic disk apparatus using the conventional PRML system, and FIG. 3, a write current waveform and a reproduction waveform by the conventional PRML system.
Referring to FIG. 2, on the recording side, write data 101 are encoded into an error correcting code signal by an error correcting code generator 111. The error correcting code signal 112 is run length limited-encoded (RLL-encoded) by an error correcting code signal generator 113, and a RLL code 114 is thereby generated. A read auxiliary signal generator 12 consists of a preamble signal generator 121 and a synchronizing signal generator 123, and the preamble signal generator 121 generates a preamble signal 122 having data clock information and amplitude compensating information. The synchronizing signal generator 123 generates a synchronizing signal 124 containing read clock information. The RLL code 114, the preamble signal 122 and the synchronizing signal 124 are arranged by a signal arrangement circuit 125 into a sequence fitting a format.
A write compensation circuit 131 generates a compensated write signal 132 having undergone compensation for the component of distortion to which the signal is subjected on a recording medium, and the signal is amplified by a write amplifier 133. A write head 135, which is a magnetic head, writes information on a recording medium 103 on the basis of a write current waveform 134. The write current waveform manifested by the recording signal 134 will be described afterward with reference to FIG. 3.
Then, on the reproducing side, information written on the recording medium 103 is read by a read head 151 to obtain a read current signal 152. A read amplifier 153 amplifies the read current signal 152, and delivers it to an equalizer 163 via a read compensation circuit 161. Here, a read auxiliary circuit 165 extracts the preamble signal 122 and the synchronizing signal 124. It further extracts read clock information from the synchronizing signal 124, and extracts data clock information 167 and compensated amplitude information 167 from the preamble signal 122.
By utilizing these read auxiliary signals, the equalizer 163 supplies an equalized signal 164 whose waveform is shaped to desired partial response characteristics. This equalized signal 164 is entered into an ML decoder 171, which delivers a partial response signal 170. A RLL coder 181 acquires an RLL signal 182 from the partial response signal 170. An error correcting code decoder 183 corrects any error in the RLL signal 182 to acquire read data 102.
Next will be described the write current waveform 134 and the read current signal 152 of the PRML system with reference to FIG. 3. In a conventional recording system, such as the PRML system, information is determined by whether or not the amplitude of the write current waveform 134 is inverted according to the recording bit that is entered. When xe2x80x9c1xe2x80x9d is entered, the current waveform is inverted (191), or when xe2x80x9c0xe2x80x9d is entered, the state before its entry is maintained, and the current waveform is not inverted (192). If xe2x80x9c1xe2x80x9d is consecutively recorded (194), the current waveform will be inverted in every bit period, and the inverting intervals of the write current will be minimized (193). The value that this write current waveform 134 can take is either one of positive and negative levels (xc2x11), and the amperage varies in every bit period. Therefore, information that can be recorded per bit period is one bit.
For read signals on the other hand, the minimum inverting interval of magnetization becomes shorter with an increase in density and, where the adjacent magnetization is inverted (194), the read signal is much weakened by interference (195). Moreover, the higher the density, the greater the impacts of medium noise and thermal demagnetization, giving rise to a problem that magnetization is lost and errors increase.
Signal processing techniques applicable to such a magnetic disk apparatus include improved versions of the PRML system, such as the extended PRML (EPRML) system and the expanded EPRML (EEPRML) system. These system effectively utilize the energy of signals, weakened by interference, by expanding the energy per bit over the time of delay during which the interference occurs.
Other techniques for expressing signals at multiple levels and recording/reproducing them include a multi-level modulation recording system using an orthogonal modulation technique. There is a system by which information is divided into in-phase and quadrature components and modulated, and combining the so-modulated components makes possible recording of multiple levels. This technique is disclosed in the Japanese Patent Laid-open No. 6-325493. According to this patent application, write information is divided into two signal sequences, of which one is not encoded and the other is convolutionally encoded. Each signal sequence is entered into a circuit known as a signal mapper, and the signals are arranged at quadrature points so arranged on a circle as to maximize the distances between the signals. After the arrangement of signal points, a carrier frequency referencing a system clock is modulated with sine components and cosine components. According to this technique, the resultant modulated waveforms are quantized at a plurality of levels, and signal waveforms having undergone digital-to-analog (D/A) conversion are recorded.
On the other hand, as one of multi-phase quadrature angular modulation systems, there is the continuous phase modulation (CPM) system disclosed in a book by J. G. Proakis and elsewhere.
By the CPM system, information is expressed in phase difference and frequency difference. The modulation waveform of this CPM system, unlike those of usual modulation systems, becomes continuous in symbol periods, has no steep variations. Accordingly, it allows narrowing of the frequency band of the modulation waveform. Therefore, it is known as a modulation system for communication apparatuses including those for wireless communication with a view to enhancing the efficiency of frequency utilization. The prior art in this category includes techniques for enhancing the efficiency of frequency utilization or use for modulation in communications as discussed in J. G. Proakis, Digital Communications, 3rd edition, pp.190-301, 1995 (first published in 1989). More recently, a combination of differential detecting and Viterbi decoding is disclosed in the Japanese Patent Laid-open No. 9-289529 and elsewhere regarding a technique for use in communication apparatuses as a CPM demodulating method in satellite communication and other situations where sufficient accuracy is not ensured.
Modulation and demodulation in a communications apparatus use a carrier wave. A communications apparatus may use as its carrier either the cosine wave (cos(2xcfx80fct) where fc is the carrier frequency and t, the time) of the in-phase component expressed as a real value or exp(j2xcfx80fct) (j expresses an imaginary unit) indicated by Euler""s theorem expressed as an imaginary company besides the in-phase component. Where a carrier of exp(j2xcfx80fct) is used, a satisfactory S/N ratio can be obtained, unaffected by harmonic components.
According to the conventional recording technique, since signals are expressed in terms of the presence or absence of bit inversion in every bit period, the minimum interval of the inversion of magnetization is determined by the bit period. Therefore, with an increase in recording density, the bit period shortens and so does the minimum interval of the inversion of magnetization. This shortening of the minimum interval of the inversion of magnetization invites a deterioration in S/N ratio.
In spite of this problem, higher-order PRML systems such as EPRML and EEPRML are nothing to improve the minimum interval of the inversion of magnetization. On the other hand, according to the technique disclosed in the Japanese Patent Laid-open No. 6-325493, the amplitude inversion of the recording current at a frequency of an integral multiple of the bit period because the convolutional encoder and the carrier frequency reference a system clock. Or in an apparatus in which the amplitude is made discrete and the amplitude level that can be taken is fixed, the level of quantization is limited. These techniques cannot enhance the density of recording in an apparatus that can take only two levels (xc2x11) of amplitude, such as a magnetic disk apparatus, and the interval of the inversion of magnetization cannot be extended.
Recording of two kinds of signals, in-phase and quadrature, would reduce the density of recording, and therefore is not applied to recording apparatuses. Moreover, as no consideration is given to solving the problem of a reduced recording density, there are no signal processing functions unique to recording apparatuses, such as controls on the discreteness of amplitude and the bit inversion interval, there is no precedent of applying the CPM system to a recording apparatus. Nor is there any case of improving the recording frequency band by the CPM system.
An object of the present invention is to provide a signal processing method for enabling an information recording and reproducing apparatus, in which the amplitude signals can take is limited, to apply signals of multiple levels beyond this amplitude to recording and reproduction.
Another object of the invention is to provide a signal processing method for enabling, where information is to be expressed in phase difference or frequency difference, the bit inversion interval of recorded signals to be controlled by regulating the timing of amplitude variation.
According to the invention, in an information recording and reproducing apparatus which makes the amplitude of signals discrete and whose recorded/reproduced signals have a fixed amplitude and a fixed level, data are expressed at the time at which signals vary, and the interval of signal inversion is controlled to a certain fixed length. For this reason, a phase/frequency modulation/demodulation system, which provides a continuous modulated waveform, to make the amplitude of the modulated wave discrete.
Where it is to be applied to a magnetic disk apparatus, as a magnetic disk apparatus can take two levels of amplitude, the amplitude has to be made discrete. For correct demodulation of signals resulting from making the amplitude discrete, a modulation system which would keep the envelope constant is used. As one of such modulation systems, the CPM system known as a narrow band modulation system, whose envelope is constant, is used. Since the CPM system performs narrow band modulation, the interval of signal inversion can be limited within a certain range. Furthermore, because information is expressed in the phase and frequency components of signals, even where the amplitude is made discrete and signals recorded/reproduced have a certain fixed amplitude level, signals that can be distinguished at multiple levels can be recorded and reproduced.
The invention also achieves a signal processing method for use by an information recording and reproducing apparatus which, in demodulating signals to be reproduced by using a demodulation system requiring frequency characteristics similar to the characteristics of the transmission channel, varies the power ratio between the upper side band and the lower side band according to the recording density thereby to suppress noise in the upper side band when accomplishing demodulation.
For this purpose, there is configured a demodulating system taking account of power distribution between the upper side band and the lower side band in which signals to be recorded and reproduced are deformed by the characteristics of the transmission channel of the recording/reproducing system. More preferably, a vestigial side band (VSB) demodulating system should be used. Whereas the characteristics of the transmission channel given by the recording/reproducing system of a magnetic disk apparatus, for instance, significantly attenuate the upper side band, the use of a VSB demodulating system according to the invention makes it possible to suppress noise in the upper side band and improve performance without having to increase the circuit dimensions by utilizing the characteristics of the transmission channel that the upper side band is significantly attenuated and introducing a means of equalization to the VSB demodulation characteristics.